Polychlorinated biphenyls (PCBs), 2,4,5,2',4',5'-hexabromobiphenyl (PBB), and Kepone are readily absorbed from the gut, rapidly removed from the blood, and initially stored in the liver and muscle. At later time points the less chlorinated PCBs are metabolized and excreted while the highly chlorinated PCBs and the PBB are translocated from the liver and muscle to the skin and adipose tissue which are the sites of long-term storage. The degree and rate of metabolism and the ultimate biologic half-life of PCBs is determined by the degree and position of chlorination. The degree of PCB metabolism decreases as the chlorine content increases, and the effect of increasing chlorine content is most marked when the chlorine atoms are arranged so that the biphenyl molecule does not have two adjacent unsubstituted carbon atoms. The PBB and Kepone do not have two adjacent unsubstituted carbon atoms and are not subject to an appreciable amount of metabolism. Kepone is stored primarily in the liver and is excreted in the bile at a rate proportional to the liver concertration. On the other hand, the PBB is stored primarily in the adipose tissue, is subject to very little excretion, and thus has an infinite half-life in the male rat.